Drosophila melanogaster alcohol dehydrogenase: product-inhibition studies.

The Drosophila melanogaster alleloenzymes AdhS and AdhF have been studied with respect to product inhibition by using the two substrate couples propan-2-ol/acetone and ethanol/acetaldehyde together with the coenzyme couple NAD+/NADH. With both substrate couples the reaction was consistent with an ordered Bi Bi mechanism. The substrates added to the enzyme in a compulsory order, with coenzyme as the leading substrate, to give two interconverting ternary complexes. The second ternary complex broke down with release of products in an obligatory order, with the aldehyde/ketone leaving first. Both the acetaldehyde and acetone products formed binary complexes with the enzyme that affected NAD+ binding. However, only an enzyme-acetone complex seemed to affect NADH binding and hence the reverse reaction. The inhibitory pattern with acetaldehyde as product was also affected by the formation of a ternary enzyme-NAD(+)-acetaldehyde complex, which broke down to acetic acid and NADH. The product-inhibition pattern shown in the present work is different from that published for Drosophila Adh previously and this discrepancy can not be explained by the use of different variants of Drosophila Adh.